The present invention relates to an arrangement for limiting the drive ratio of a variator in a continuously variable transmission.
In any continuously variable transmission, such as a motor vehicle transmission, there is a device, referred to herein as a “variator”, which is responsible for providing the continuous variation of drive ratio. Variators take numerous different forms. For example, some existing automobile transmissions use “belt and sheave” type variators, in which the belt runs on expanding pulleys or sheaves. Another well-known type of variator has one or more pairs of co-axially mounted races, which are shaped to form, together, a toroidal cavity. Drive is transmitted from one race to the other by a set of rollers disposed within the cavity and running upon the races. The inclination of the rollers is variable to change the variator's drive ratio.
The variator typically requires some mechanism, to be referred to herein as an “end stop”, for ensuring that the variator is not driven beyond the range of ratios it is able to provide, which could otherwise result in damage. For example, in the toroidal-race type of variator. if the drive ratio, and correspondingly the roller inclination, were to change beyond the acceptable range, then the rollers could be caused to leave the toroidal races altogether, resulting in catastrophic failure. The end-stop function is particularly important in the “torque controlled” type of variator in which ratio is permitted to vary under the influence of torques at the transmission input and output.
It might be imagined that a simple mechanical stop on the travel of the relevant part(s) —on the rollers' inclination, or the pulleys' expansion, for example—would serve the purpose. However this is typically not an ideal solution. To understand why, it is necessary to appreciate that within the variator a balance is required between the torque that the variator is handling and a contact force with which torque-transmitting elements, such as the belt and pulleys, or the rollers and discs, are biased together. If this biasing force is too great then efficiency suffers and wear is unnecessarily increased. If it is inadequate then slippage can take place, which is inefficient but also potentially damaging. The required force varies with the torque being handled, so it is usual to provide some means for varying the biasing force in sympathy with torque. In the toroidal-race type of variator, this can be done through a hydraulic arrangement in which both the biasing force and a reaction force applied to the rollers depend on a common hydraulic pressure. This type of arrangement is described, for example, in Torotrak (Development) Ltd.'s published international patent application PCT/GB2004/002139 (publication number WO2005/015059). The problem created by a mechanical stop is that when it takes effect, a sudden change can be created in the torque upon the variator. Unless the biasing force is correspondingly adjusted, there is a danger of slippage within the variator.
In the type of hydraulically controlled toroidal-race variator just mentioned, the end stop function can be achieved hydraulically, as described in the international patent application just mentioned. Here, the reaction force is applied to the rollers by piston/cylinder arrangements. Supply of the working hydraulic fluid to the cylinders involves side ports that are closed by the pistons when they reach the end of their travel, producing a hydraulic lock to arrest the movement of the pistons and of their associated rollers. The resulting increased hydraulic pressure in the cylinders is passed on to a hydraulic ram providing the biasing force, which is thus automatically increased as necessary.
While highly effective, such end stops are not universally applicable and involve some complication in the controlling hydraulics. An alternative form of end stop is thus desired.
US patent application US2003/0083170, Ooyama, describes a transmission in which a chain drive couples the variator input to a one-way clutch whose other part is coupled to the variator output. The clutch engages to prevent transmission ratio exceeding a chosen maximum. The arrangement involves additional constructional complexity due to its use of a chain drive to provide the required ration and drive transfer from the transmission input to the one-way clutch.
The present inventor has recognised that in certain transmissions a one-way clutch can be used to provide a variator ratio end stop function in a more straightforward manner.